The formation of very high contrast photographic images using certain types of silver halides is known, and such methods of forming photographic images are used in the field of photographic plate making.
Special developing solutions, called lithographic developers, are used for these prior objectives. Lithographic developers contain only hydroquinone as the developing agent. In order to prevent infectious development, sulfite as a preservative is used in the form of an addition product with formaldehyde, and the concentration of free sulfite ions is made very low (ordinarily below 0.1 mol/l). Because of this, a lithographic developer undergoes air oxidation very easily, and has the great deficiency of being not preservable more than 3 days.
Methods for obtaining high contrast photographic properties using stable developers include methods using hydrazine derivatives described in U.S. Pat. Nos. 4,224,401, 4,168,977, 4,166,742, 4,311,781, 4,272,606, 4,211,857 and 4,243,739. These methods give high sensitivity with supercontrast, and permit additions of high sulfite concentrations in the developer. This markedly raises its stability against air oxidation, compared to lithographic developer.
However, these supercontrast image forming methods using hydrazine compounds have problems in that, in order to promote strong infectious development, the parts that should have been narrow white places, when photographing low contrast manuscript writing (particularly the fine lines of Ming-style characters), are blackened so that the characters became smudged with black, and are difficult to discern. Because of this, when the exposure was lowered to match the fine lines of the Ming-style characters, reverse problems occur in that smudging of Gothic letters worsen. Thus the latitude of exposure is narrow. Similar problems also occur in photographing dot images, where darkening tends to occur in the parts left as white space among the dots, and half tone gradation becomes very narrow.
This is caused by the fact that low exposure parts or unexposed parts adjoining the exposed parts end up being developed, because of the strong infectious developability with supercontrast, resulting from the hydrazine compounds. In order to prevent this, development of a method for suppressing image expansion due to infectious development is desired, and development of a method of bringing about a development effect that inhibits development of the parts adjoining the exposed parts (hereafter micro development inhibition) is also desired. Suppressing infectious developability can be attained by reducing the amount of nucleating agents added and lower the pH of the developer. However, practical problems occur in that the tone becomes soft, and line-image sharpness is lost. Also, although various investigations into imparting micro development inhibition in nucleating development systems have been made, no satisfactory method has yet been discovered.
The present invention accomplishes control over nucleating development by using the developer at a pH below 11.2, that is, it accomplishes image expansion and micro development inhibition. Conventionally, when the pH is below 11.2, high contrast images having G (gamma value) of 8 or more cannot be formed, although sufficiently high contrast can be obtained by the combined use of a nucleating accelerator as disclosed in JP-A-63-183438. (The term "JP-A" used herein means an unexamined published Japanese patent application.)
Also, with development at a pH of less than 11.2, obviously infectious developability is weaker and image expansion is smaller in comparison with contrast development at higher pH, and development of the unexposed parts or low exposure parts of the adjoining segments of the image is microscopically controlled. As a result of further analysis, this development effect has been understood as being closely related to the crystal habit of the silver halide.
On the other hand, while it is easy to obtain supercontrast images having G of 8 or more at a pH over 11.2, the stability of the developer worsens, the development agent oxidizes and deteriorates, the pH fluctuates because of the absorption of CO.sub.2 from the air, and the photographic characteristics often change. Further the development effects as observed below pH 11.2 cannot be obtained, and the latitude of exposure amounts to provide both Ming-style characters and Gothic characters with satisfactory image qualities is narrow.